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Because as I mentioned mate its a figure based on air at atmospheric pressure, so if you have a bar of boost you can fit in 800cc of air based on atmospheric pressure to a 500cc cylinder pretty easily, so therefore its 160% efficient

So say for example a N/A 2.0 16v makes 150 bhp add one bar of boost and the MAX it can make is 300 surely...

Thats a little over simplified, after all, what would be the manifold pressure on that 2.0 16v at 150bhp? I would wager minus a couple of psi at least.
Otherwise how could throttle bodies improve anything?

So looking at absolute pressure, you are talking 12-13 psi probably, so at 29psi (1 bar of boost) its easily possible to put more than double in, as you have effectively got about 2.3 times the pressure differential

Then when you consider that the engine wont get below a couple of psi at BDC anyway as it didnt empty its cylinder totally, you realise you are actually not talking about 12psi differential, but 10, and your 29 represents 27, so nearly 3 times the amount.

Then consider that was all at BDC, and most of the cycle the piston isnt, and you will see even more difference!

theres a point in out cycle when we want peak cylinder pressure to happen to apply the most amount of force on our crank

but sadly petrol has a pressure to which it will "knock" at so sometimes if we are running a compression ratio too high fo our petrol we have to set or ignition advance not set at the optium to avoid detenation issues of our petrol (im not saying this is always the case aswell)

its a balancing act, between cylinder pressures/avioding knock and producing the most amount of force on our crank at the right crank angle

I understand that cylinder filling changes due to different condtions, revs being one.

people think tuning an engine is easy

500 bhp= t4 turbo and bd cams...if only it was that easy

Glad to see that we are exposing exactly what goes on when tuning an engine! I think with better understanding of what goes on in an engine, mistakes and bad engines can be avoided!

Actually mate, its more likely to do the opposite, forums can be bad at empowering people to the point they *think* they know enough to build their own engine, then find out the hard way they didnt

Quote:

Originally Posted by JjCoDeX75

Back to low compression a mo (sorry!). I'm not sure I understand what about the lower compression allows for greater advance. Does it relate to the heat generated in the combustion?

It kind of relates to the heat yes, if you try and ram 2 bar of boost into a high comp engine you end up with massive amounts of ignition retard (or tiny advance if you look at it that way) to avoid DET.

This becomes an issue more if the car is used hard for long periods, which is why you see phil and mike disagreeing on it, as they use their cars very differently, you can "get away with" more if the car isnt used at WOT for long

The basic effect of this retard (the timing, not phil) is to make the point at which peak cylinder pressure are greatest move later in the stroke.
This has several effects.
Firstly, it potentially loses you power if you go too high comp, as the point you want to be pushing hardest at you are still only halfway through the burn and secondly and more dramatically it results in a drastic rise in EGTs as some of the energy that would have been used to push the piston down is instead released so late that it instead just heats the gasses up, the third noteable effect therefore of course is another loss in power from that happening.

People always says "higher comp means more power from the same boost" but thats actually a sweeping generalisation that doesnt always hold true, and almost never holds true if you are talking big boost engines and looking at the figures near peak torque, so while it may increase power at the top end, it will often lose you torque even if the boost stays the same, something people's limited understanding doesnt allow for them grasping.

You will notice that I seem to have contradicted myself there, I just said that going high comp the negative effect is losing you power, but then I just stated that at the top end it can gain you power, the reason for this is that at the top end you have such massive advance figures that the peak cylinder pressures can actually fail to be as high on a low comp engine as they are on a high comp engine whilst the high comp still manages (at that point) to make them at a decent point in the cycle.

So again, its a case of compromise, generally speaking for a big boost engine, lower comp works better in the midrange, and higher comp works better at the top end.

my fav line of a tuner i once talk to was "we can now use high compression because of modern ecus"

what i couldnt ask for 5 degrees of advance before

Thats weird, I actually AGREE with the statement that moden ECU's allow you to use both higher and lower compression than you could before.

Must be the first time me and you have disagreed on something so fundamental on here IIRC!

My reasoning is that higher resolution allows you to be more accurate with the mapping, and at the extremes of CR that is particuarly important, however I very much doubt I agree with it to the extent the person who said it to you does from how you are wording that.

But giving your example of 5 degrees, you could ask for 5 degrees before, but you got it in LOTS of places, which meant you ended up having to dial in less advance than you wanted in areas of the graph just cause you didnt have the resolution to do it accurately enough at all the points you really want to.

i cant get over phil in this thread, over confidence, dillusions, or taking the piss, im not sure, but its at least one of them as he spouting utter crap most the time and then thinking this thread would be nothing without him

I remember a while ago, someone talking about turbo engine capacity, and the actual "capacity" of the engine is exponentially increased as a result of having the ability to make as well as consume air.

Is that correct?

Really interesting post, and if a mod did the decent thing and chopped Phil's replies out, it would be even better.

Right - I think I have it - In Jamesspeak, if the compression ratio is higher, then the fuel is compressed more. There is a point where that compression leads to knocking as you efectively cand retard the ign enough to compensate ???

Right - I think I have it - In Jamesspeak, if the compression ratio is higher, then the fuel is compressed more. There is a point where that compression leads to knocking as you efectively cand retard the ign enough to compensate ???

JJ

Kind of, but bear in mind that its more about the burn, and the extra gas that releases and how that interacts with the compression ratio, even at 20:1 you wouldnt see det from comrpession alone.

Right - I think I have it - In Jamesspeak, if the compression ratio is higher, then the fuel is compressed more. There is a point where that compression leads to knocking as you efectively cand retard the ign enough to compensate ???

JJ

Kind of, but bear in mind that its more about the burn, and the extra gas that releases and how that interacts with the compression ratio, even at 20:1 you wouldnt see det from comrpession alone.

Okay - I always understood the ignition advances role to ensure that the combustion occurs just after TDC (so that the piston and rod is pushed down nice and evenly, and creates optimum power). I dont think I am quite following exactly how the compression ratio affects the burn, other than if the compression ratio is lower, it had more space to create the combustion reaction.

I remember a while ago, someone talking about turbo engine capacity, and the actual "capacity" of the engine is exponentially increased as a result of having the ability to make as well as consume air.

Is that correct?

Really interesting post, and if a mod did the decent thing and chopped Phil's replies out, it would be even better.

the thread was edited to the point where if anymore was removed it might as well been deleted

We have established that for everything you change, there are generally both positive and negative reactions. The only real exception is where technology improves. At this point you can have your cake and eat it so as to speak!!!

For example, now we have lowered our compression ratio, we now need to compensate for the loss.

We have already agreed that this is done by adjusting cam timing and map.

When the cam timing is adjusted, is this to address the changes is VE?

I had a chat with Chip on the dog and bone, and he has attempted to explain the difference between various compression ratios. I will attempt to explain to you as best as I can.

Hopefully the resident experts will correct the bits that I get wrong!

The constants and assumtions

We will assume that the engine in question had a fixed number of cylinders, as well as stroke, and the number of cylinders is likewise fixed.

What is compression ratio

If I understand it, the compression ratio is effectively dictated by the amount of space at TDC. If you lower the compression ratio, then you are effectively slightly increasing this space. One of the advantages of the lower compression ratio is that it permits a greater Volumetric Efficiency (VE) (made by the effective increased area).

Please note that the volumetric efficiency is not improved by simply lowering the compression ratio - it merely allows for an improved VE if other things are changed.

The pros and cons of different ratios

First the pros - one of the most significant improvements is that the car is now able to run more advance without leading to detonation. Chip came up with a really useful way of visualising it.

Imagine that your car in std compression with say a T34 turbo. Currently it can work satisfactorily with say 1.5 bar of boost without danger of detonation (when properly set up). As you RAISE the compression ratio, eventually you will get to a point where you can no longer advance the engine enough to run that same level of boost.

The reverse and opposite is therefore true.

The second benefit is more subtle. As I understand it, a higher compression ratio car achieves power at a higher rpm. On the cosworth engine, the safe maximum realistic RPM is approx 8000rpm. Given this, when attempting to reach a certain amount of power, the compression ratio therefore needs to be lowered to allow for the intended power level to be reached.

Second, the cons.

If you simply change the compression ratio and stuff a large turbo on the side of the engine, you will increase the power lever achieved by the engine based on the above. BUT

Because the compression ratio has been reduced, at lower speeds, the engine needs to flow more air to run properly. Now given that the cams in the engine are basically set up for the factory compression ratio, they do not optimise our new - lower compression ratio.

It is because of this that the timing of the cams needs to be amended to reflect the change in the CR.

it is THIS that tends to make the difference between a good low comp engine, and a bad low comp engine.

Lets pause here, and make sure that what I have above is correct (cue all resident experts!)

I want to very briefly revisit the so called 'myth' on modern ECUs and their ability to overcome the above.

As I understand it, the argument is as follows - the more modern ECUs have more memory, and are faster. This in simple terms means that the car can store more points in a map. Effectively this leads to more accurate mapping, as there are less 'grey' areas between points.

the theory goes that the above goes that because of the above, you can more accurately plot the advance, and thereby avoid the detonation.

Having reviewed the posts on the previous pages, as well as discussing the subject a little further, it would seem to me that the problem is that the improved mapping does not allow the engine to break the rules of physics.

Effectively, whilst the modern maps may allow you to tune closer to the point of det (but not at the point of det), you will still inevitably reach a point when you can no longer advance the engine enough to compensate. At this point, it wont matter how good the ECU is.

Does this sound correct? If so, this will lead me to the next point - fuelling.

its genrally all about the size of our "look up tables" the amount of resolution we have to map with, older systems might of oly had a 8x8 look up table,,, meaning that 8 speed breakpoints ( so maybe 1000, 2000, 3000, 4000, 5000, 6000, 7000 and 8000 RPM) and 8 load sites (maybe 0, 0.375, 0.75, 1.125, 1.5, 1.875, 2.25 and 2.625 bar absoloute pressure)

as you can see the speed and load sites are far appart, we only got 64 sites in total to calibrate! and alot can happen inbween say 4000 rpm and 5000 rpm,,, but with this old ecu all we can do it calibrate for the worst case inbetween these sites

modern ecus can have much bigger look up tables

so engine calibration can be much more fine and accurate

rememebr we have two MAIN look up tables,,, one for ignition and one for fuel